And OSHA, as well as other regulatory agencies, are pretty savvy about figuring out when a discharge is retaliatory, even if a different reason is given.
The ADA may not apply. Her allergy may not turn out to qualify. The company may not be large enough to fall under the statute. Her doctor may indeed suggest different allergy meds. I'm not saying she's definitely got a claim under the ADA.
But we're a long way from telling her that her only option is to quit, which is I believe where you started out. There are still options to explore.

